The Non-Volatile Memory express (NVMe) Specification is a specification for accessing solid-state devices (SSDs) and other target devices attached through a Peripheral Component Interconnect Express (PCIe) bus. The NVMe SSD PCIe host interface defines a concept of Namespaces, which are analogous to logical volumes supported by SAS RAID (Redundant Array of Independent Disks) adapters. Copy on write functionality in an SSD may be implemented using namespaces. Namespaces are typically implemented as an abstraction above the global Logical Block Address (LBA) space tracked in an SSD's indirection system.
LBA metadata only indicates one host LBA and it does not include a reference count. Including or appending a reference count in the metadata would incur additional writes to rewrite the data with new metadata, which is a poor solution. Without such reference counts in the LBA metadata, there is not a mechanism for determining whether additional clone copies exist (e.g., that additional LBA's point to the same data). Managing multiple clone copies of data on an SSD therefore faces particular challenges with respect to garbage collection. For example, when a host modifies the ‘source’ LBA after a copy operation it may produce garbage collection challenges. The source copy may be effectively the ‘master’ copy that was written before the copy operation was performed to duplicate the data to one or more additional host LBAs. When this master LBA is modified, a non-copy-aware garbage collection algorithm may free the physical data at the next opportunity, since a method does not exist to efficiently modify that data's metadata to indicate that more host LBAs point to that data.